This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new sandwich cathode design having a first cathode active formulation sandwiched between two current collectors and with a second cathode active formulation in contact with the opposite sides of the current collectors, the active material of the first and second formulations being the same. The present cathode design is useful for high discharge rate applications, such as experienced by cells powering an implantable medical device.
Silver vanadium oxide (SVO) is known to have high power capability. However, without the use of a conductive additive, such as carbon black, graphite, etc., in an SVO cathode active formulation, its power capability at a low percent of discharge or small depth of discharge (DOD) is significantly worse than if the conductive additive were present. The problem is that the conductive additive decreases the practical density of the cathode. In other words, the gram amount of cathode active material per unit volume is lower than that of the SVO active material without the non-active carbonaceous additives.
In conventional SVO cells, the cathode active material is always mixed with a few weight percent of carbonaceous additives along with a few weight percent of binder materials. According to the present invention, SVO material without any conductive or binder additives, or with a lesser percentage of additives, is sandwiched between two current collectors. This assembly is further sandwiched between two layers of SVO material formulated having a greater percentage of binder and conductive additives than that of the sandwich assembly. As a result, lithium cells with cathodes of this configuration have the same or higher discharge rate capability as that of conventional Li/SVO cells. At the same time, the present cell exhibits equal or higher capacity than that of a conventional cell due to the greater energy density contributed by the 100% active SVO portion sandwiched in the middle of the cathode. Higher cathode efficiency is also realized with this cathode design.
Accordingly, one object of the present invention is to improve the performance of lithium electrochemical cells by providing a new concept in electrode design. Further objects of this invention include providing a cell design for improving the capacity and utilization efficiency of lithium-containing cells.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.